Preheat oven to 375F (190C). Crush cookies into small pieces. You should have about 3/4 cup (175 mL). Place in a medium bowl. Stir in flour and brown sugar. Cut in butter with 2 knives or your fingers until mixture is crumbly.
In a large bowl, use a fork to stir granulated sugar with cornstarch, cinnamon and nutmeg. Peel mangoes and cut flesh away from pit. Slice into 1-inch (2.5-cm) chunks. Add to bowl and toss. If using fresh rhubarb, cut off and discard leaves. If using frozen rhubarb, rinse under cold running water to melt ice crystals, then pat dry with paper towels. Toss rhubarb with mango mixture. Then turn into a buttered, 8-inch (20-cm) baking dish. Sprinkle cookie mixture over fruit but do not press down. Set dish on a large, rimmed baking sheet.
Bake in preheated 375F (190C) oven until fruit is tender and topping is golden, 45 to 50 minutes. If necessary, cover loosely with foil during last 15 minutes of baking to prevent overbrowning. Serve warm with vanilla ice cream. Any leftovers will keep well, covered and refrigerated, up to 2 days.
Nutrition (per serving)
Crumbled cookies, fresh ruby-red rhubarb and juicy mango make an irresistible crisp.